A storage servo system is used to reduce mechanical displacement mismatches between an actual position of a read/write head and a target position of the head. The mechanical displacement mismatches, or disturbances, in a servo system may include RRO and non-repeatable runout. Repeatable runout may be caused by disc irregularity, disc eccentricity and/or spindle axis assembly (mechanical misalignments) and are repeatable in each spindle rotation. Non-repeatable runout is not synchronized with disc sectors, e.g., unmodeled dynamics.
Some hard disk drives (HDDs) use a frequency domain multiple sine wave synthesizing method for feed-forward RRO disturbance control. This method needs significant processing power to generate multiple sine waves and calculate the phase and magnitude for each sine wave to compensate for repeatable runout.
A removable disc is not fixed to any storage system and may be loaded into a storage system when needed. Examples of removable disc storage systems may include CD-ROM drives and DVD drives. Some people believe that disturbances in removable disc storage systems are not predictable, and that repeatable runout control (RROC) in removable storage systems is impossible, because the recording media is removable and the loader mechanism is low cost. Others are trying to implement RROC in removable disc storage systems using the above-described magnetic storage approach, using a high bandwidth feedback control loop with a high sampling rate. However, with the high sampling rate, such an approach is prohibitively expensive.
Therefore, it may be desirable to provide a method and apparatus for reducing RRO in removable disc storage systems.